This invention generally relates to an image forming apparatus and, more particularly, to an image forming apparatus capable of selective halftone control using a thermal transfer image forming medium such as an ink ribbon.
In recent years, as a method of obtaining a halftone image, an areal gradation method using a meltage thermal transfer ink ribbon and a density gradation method using a sublimation thermal transfer ink ribbon have developed. However, the density of each dot is not changed in the areal gradation method. In the density gradation method, the density is changed for each dot, so that halftone control can be achieved in units of dots. For this reason, a copying machine using the density gradation method has good resolution and can print an image clearer than that obtained by a copying machine using the areal gradation method. However, according to the density gradation method, printing time is undesirably prolonged. Therefore, a copying machine using the density gradation method is effective in a case wherein clear copies are desired even if a printing speed is low, but such a copying machine is not practical.
Accordingly, a demand has arisen for a system which can use the areal gradation method when a printing speed is more important than image quality as halftone control performance, and can use the density gradation method when the image quality is more important than the printing speed.
In recent years, graphic arts printing has developed markedly along with an advance in electronics, and electronic graphic arts printing has become very popular. In electronic graphic arts printing, the density of an image of an original is photoelectrically converted into an electrical signal and the electrical signal is subjected to predetermined processing to obtain a light intensity signal. The intensity signal is used to expose a film (photosensitive medium) with light. The exposed film is developed to prepare a negative or positive film for an original plate.
According to the above technique, however, the printing process is complicated and printing equipment is bulky, thus resulting in high costs. A conventional system is designed to produce only graphic arts films and cannot be used for other applications.
In order to align graphic arts films (block copies) of the respective colors, positioning marks (so-called register marks) are manually adhered to graphic arts films, thus degrading operability and causing potential positional shifts.
Accordingly, a demand has arisen for a system which can be used not only to form graphic arts films but also as, e.g., a color copying machine.
In this case, selective halftone control as described above is desired again. However, positioning marks must be easily and reliably adhered to the graphic arts films as described above. Therefore, it is necessary to reconsider halftone control for the positioning marks.